Synthmaster is a different beast, has a lot more options (different synthesis engines) etc. and this cannot be compared with Serum. I must also add that Serum has a serious automation bug which is still not fixed (I wrote with Steve, made a video... but it's still there).
Serum is one of the best wavetable synth (IMO) but.. it's "only" a Wavetable synth and yes, people saying very often it has a bit cold/clean sound. So it depends of the own needs (but to be honest, I don't know if I would buy Serum again...)

